Optical amplifier and an optical amplification method

ABSTRACT

Two rare earth-doped optical fibers are connected in series and used to amplify input light. A splitter is installed between these two rare earth-doped optical fibers. The input light is monitored by having the portion of the input light that is branched off by the splitter received by a photodiode. Excitation light output from a laser light source is guided by optical couplers and supplied to the above rare earth-doped optical fibers. A control circuit controls the output light level and, at the same time, stops the output from the laser light source when the input light level drops below a specified threshold value. The gain of the first stage rare earth-doped optical fiber while excitation light is being supplied is larger than the loss that occurs due to branching of the input light by the splitter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical amplifier. In particular, itrelates to an optical amplifier that has a function for monitoring aninput light level.

2. Description of the Related Art

The amount of information sent and received via networks is increasingrapidly. In addition, as the information becomes more international incharacter, the importance of long-range communications is rapidlyincreasing. In this kind of long-range transmission, particularly when alarge amount of information is transmitted, optical fiber cables areused. However, when signals are transmitted via optical fibers, as thetransmission distance increases, the signal is attenuated. For thisreason, in long-distance optical transmission, relay nodes containingoptical amplifiers are normally installed at specified intervals. Thesignal light is amplified at each node and then sent to the next node.

A number of types of optical amplifier have been developed. One of thetypes is the optical fiber amplifier. In particular, in the 1550 nmband, optical amplifiers using rare earth-doped optical fiber, intowhich rare earth elements such as erbium have been doped, are widelyused. In a rare earth-doped optical fiber, the rare earth elements areexcited by excitation light which is input apart from the signal light,and the signal light passing through the optical fiber is amplified bythe excitation energy.

Some optical amplifiers have a mechanism to monitor the input lightlevel. That is to say, the input light level is monitored and, whetheror not its level drops below a threshold level is monitored. Conceivablereasons why the input light level might drop below the thresholdinclude: (1) Light is not being output from the sending side, or is notarriving for some reason, such as the optical fibers are broken, (2)Although light is being output from the sending side, that light doesnot include a signal that contains information to be transmitted.

Thus, since, if light or a signal is not being transmitted, it is notnecessary for the optical amplifier to amplify anything, the excitationlight that excites the rare earth elements is stopped. This unitcomposition conserves the power used to drive the light source (normallya laser) that outputs the excitation light. In addition, since theamplification action inevitably generates noise, stopping theamplification action prevents the optical amplifier itself from becominga noise source.

In an optical communication system that uses optical amplifiers, thenoise that is generated in the different amplifiers accumulates. Forthis reason, in particular in a transmission area where many opticalamplifiers are connected in series, it is necessary to suppress theamount of noise generated in each optical amplifier as much as possible.The amount of noise generated in an optical amplifier is expressed asthe S/N (signal-to-noise ratio) of the output light relative to the S/Nof the input signal, and is called the noise index.

In an optical amplifier having the above composition, the mechanism thatmonitors the input light level is one of the causes that prevents thenoise level from being reduced. That is to say, in order to monitor theinput light level, normally an optical splitter, for example, is used tobranch off part of the input light; then the level of this branched-offpart of the input light is detected by, for example, a photodiode, andthe input light level is computed. For this reason, part of the inputlight is lost without being transmitted to the output side, causing thenoise index to become worse. In particular, for example, if thetransmission path on the input side is long, when the input light levelis low, in order to measure that light level accurately, the amount oflight that is branched off to the photodiode side must be kept at orabove a certain fixed level, thereby decreasing the amount of light thatis actually amplified for transmission and making the noise index evenworse.

In order to deal with this problem, a configuration in which the inputlight level would be monitored indirectly has been proposed. That is tosay, when the input light includes a signal, the input light isamplified while the excitation light power used to excite the rare earthelements is held fixed; the output light is branched off, and the levelof the branched light is measured to compute the input light level.However, in this configuration, if the excitation light is stopped toconserve power when the input light does not include a signal, the inputlight is passing through an optical fiber which contains unexcited rareearth elements, in which case the attenuation (loss) will be large. Forthis reason, when the input light switches from not containing a signalto containing a signal, there is a danger that the monitoring mechanismon the output side of the optical amplifier will not be able to detectthat change of condition. In this case, the excitation light is notoutput and the input light cannot be amplified. Consequently, in aconfiguration in which the input level is indirectly monitored bymeasuring the level of light output from the optical amplifier, it isnecessary to constantly supply excitation light to the rare earth-dopedoptical fibers, preventing power from being conserved.

SUMMARY OF THE INVENTION

The aim of this invention is to provide an optical amplifier in whichboth power consumption and noise are low.

The optical amplifier of this invention is configured so as to controlan amplification action depending on the input light level. It has aninput monitor and first and second optical fiber amplifiers. The inputmonitor monitors the input light level. The first optical fiberamplifier is provided on the input side of the input monitor andamplifies the input light with a gain more than enough to compensate forthe loss due to the input monitor. The second optical fiber amplifier isprovided on the output side of the input monitor; it amplifies and thenoutputs light that has passed through the input monitor. The first andsecond optical fiber amplifiers both consist of optical fibers dopedwith rare earth elements.

The optical amplifier of this invention can be configured so as toadditionally include a light source that supplies excitation light tothe first and second optical fiber amplifiers, and a light sourcecontrol unit which either stops the light source or reduces the outputof that light source when the input light level detected by the inputmonitor is lower than a preset threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the principle of this invention.

FIG. 2 is a schematic diagram of a configuration of an optical amplifierthat is one embodiment of this invention.

FIG. 3 is a graph showing the states of the input light level and thelaser light source.

FIG. 4A is a graph showing the dependence on length of the gain of therare earth-doped optical fiber.

FIG. 4B is a graph showing the dependence on input light level of thegain of the rare earth-doped optical fiber.

FIG. 5 is a block diagram showing one example of a system configurationin which the optical amplifier of this configuration is used.

FIG. 6 is a detailed block diagram of the control circuit.

FIG. 7 is a diagram explaining the control circuit action.

FIG. 8 is a diagram explaining changes of the state of the opticalamplifier.

FIG. 9 is a schematic diagram showing a configuration of an example of amodification of the optical amplifier of this embodiment (No. 1).

FIG. 10 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 2).

FIG. 11 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 3).

FIG. 12 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 4).

FIG. 13 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 5).

FIG. 14 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 6).

FIG. 15 is a schematic diagram show ing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 7).

FIG. 16 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 8).

FIG. 17 is a schematic diagram showing a configuration of an example ofa modification of the optical amplifier of this embodiment (No. 9).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram showing the configuration of the principle of theoptical amplifier of this invention. The optical amplifier of thisinvention is required to be constructed so that the amplification actionis controlled in accordance with the input light level.

The input monitor unit 1 monitors the input light level to this opticalamplifier. The input monitor 1 branches off part of the input light andthen computes the light level of the input light from the measured levelof that branched-off portion of the input light, so that a certainamount of loss occurs.

The first optical fiber amplifier 2 consists of, for example, a rareearth-doped optical fiber. It is located on the input side of the inputmonitor unit 1, and amplifies the input light with a gain that isgreater than the loss due to the input monitor unit 1. The first opticalfiber amplifier 2 is an optical amplifier for the purpose ofcompensating for the loss that occurs in the input monitor unit 1. Thesecond optical fiber amplifier 3 consists of, for example, a rareearth-doped optical fiber; it is located on the output side of the inputmonitor unit 1, and amplifies and then outputs the light that has passedthrough the input monitor unit 1. The second optical fiber amplifier 3is an optical amplifier for the purpose of amplifying the output lightto the desired level.

In the above configuration, the input light level can be monitoredwithout causing loss on the input side of the second optical fiberamplifier 3.

In the above optical amplifier, there is (1) a light source thatsupplies excitation light to the first optical fiber amplifier 2 and thesecond optical fiber amplifier 3, and (2) a light source control unitthat halts or reduces the output from the light source when the inputlight level detected by the input monitor unit 1 is lower than a presetthreshold value. This keeps the loss in the first optical fiberamplifier 2 less than the difference between the minimum light levelprescribed for the system in which this optical amplifier is installedand the minimum light level that can be detected by the input monitorunit 1 when excitation light is not being supplied.

In this configuration, even when excitation light is not being suppliedto the first optical fiber amplifier 2, if the input light level isequal to or greater than the minimum light level prescribed for thesystem in which this optical amplifier is installed, the input monitorunit 1 can detect the input light level.

Now an embodiment of this invention will be explained. The opticalamplifier of this embodiment is constructed so that input light isamplified using an optical fiber amplifier, excitation light supplied tothe optical fiber amplifier is controlled to obtain light of the desiredpower. We discuss a rare earth-doped optical fiber as one form of anoptical fiber amplifier. In addition, this optical amplifier has afunction which monitors the input light level, and, when that inputlight level falls below a threshold value, judges that the input lightdoes not contain a signal (or that light is not being transmitted to theamplifier) and halts (or reduces) the excitation light to conserveelectric power.

FIG. 2 is a diagram of the configuration of the optical amplifier of oneembodiment of this invention. In FIG. 2, the "X" marks show locationswhere optical fibers are fused together. It is also possible to useoptical connectors or lens couplers to guide the light from one opticalfiber to another instead of fusing the optical fibers together. Anoptical amplifier of this embodiment could, for example, be installed ina system which uses light in the 1550 nm band to transmit signals.

The rare earth-doped optical fibers 11 and 12 are optical fibers intowhich a rare earth substance such as erbium has been doped. Whenexcitation light is supplied from the laser light source 13 to the rareearth-doped optical fibers 11 and 12, the doped rare earth substanceundergoes inverted excitation and enters an excited state. When signallight passes through the rare earth-doped optical fibers 11 or 12 inthis excited state, that signal light is amplified by the excitationenergy. The rare earth-doped optical fiber 11 is an optical amplifierfor the purpose of compensating for the loss that occurs in themechanism that monitors the input light level. The rare earth-dopedoptical fiber 12 is an optical amplifier for the purpose of amplifyingthe output light to the desired level.

The laser light source 13 is, for example, a laser diode in the 980 nmband or the 1480 nm band (shown as LD in FIG. 2). The output power ofthe laser light source 13 is controlled by the control circuit 19. Theexcitation light output by the laser light source 13 is wave-guided bythe optical coupler 14 and supplied to the rare earth-doped opticalfiber 12. The optical coupler 14 is, for example, a fiber type ormultilayer induction film WDM coupler (a wavelength divisionmultiplexing coupler). It couples waves in the 980 nm band and the 1550nm band, or in the 1480 nm band and the 1550 nm band. After thisexcitation light passes through the rare earth-doped optical fiber 12,it passes through the splitter 15 and is supplied to the rareearth-doped optical fiber 11.

The splitter 15 branches part of the input light (signal light) off tothe photodiode 16 so that the input light level can be monitored. Thesplitter 15 can consist of, for example, an optical coupler or a beamsplitter. The branching ratio of the splitter 15 is, for example, 10:1.The photodiode 16 receives the part of the input light that is branchedoff by the splitter 15 and converts the light level into an electricalsignal which is input to the control circuit 19.

The splitter 17 branches off part of the output light (signal light) tothe photodiode 18 in order to monitor the output light level. Thesplitter 17, like the splitter 15, can consist of, for example, anoptical coupler or a beam splitter. The photodiode 18 receives the partof the output light branched off by the splitter 17, then converts thatlight level into an electrical signal and inputs it to the controlcircuit 19.

The control circuit 19 judges the input light level based on theelectrical signal received from the photodiode 16, and judges the outputlight level based on the electrical signal received from the photodiode18. The control circuit 19 controls the laser light source 13 based onthe measured input light level and output light level. That is to say,when the input light level is higher than a certain preset thresholdvalue, the control circuit 19 drives the laser light source 13 causingthe laser light source 13 to output excitation light. At this time, therare earth-doped optical fibers 11 and 12 amplify the input light. Onthe other hand, when the input light level falls below the thresholdlevel, the control circuit 19 stops driving the laser light source 13(or drives it at a lower level). In addition, the control circuit 19holds the output light level constant, for example, by means of an ALC(Automatic Level Control) function.

Normally, the optical level (intensity) of the transmitted light ishigher when it contains a signal than when it does not contain a signal.The "threshold level" mentioned above is a reference level for thepurpose of judging whether the input light received in the input sectionof the optical amplifier contains a signal or not. This reference levelcan, for example, be preset according to the length of the input sidelight transmission path.

The connectors 20a and 20b are optical connectors which connect thelight transmission paths on the input and output sides, respectively, tothe optical amplifier of this embodiment. The optical isolators 21a and21b are installed to prevent the optical amplifier from oscillating orgoing into unstable operation due to reflections in the connectors 20aand 20b.

Short optical fibers are used for the respective connections between theconnector 20a and the optical isolator 21a, between the optical isolator21a and the rare earth-doped optical fiber 11, between the rareearth-doped optical fiber 11 and the splitter 15, between the splitter15 and the rare earth-doped optical fiber 12, between the rareearth-doped optical fiber 12 and the optical coupler 14, between theoptical coupler 14 and the splitter 17, between the splitter 17 and theoptical isolator 21b, and between the optical isolator 21b and theconnector 20b. These optical fibers cannot be bent sharply and eachoptical fiber has to undergo processing of excess length, so,considering the problem of fitting them into the available space, it isdesirable for the number of optical fibers that have to be installedinside the optical amplifier to be kept small. If, for example, amongthe optical fibers listed above, the optical fibers which are providedbetween the rare earth-doped optical fiber 11 and the rare earth-dopedoptical fiber 12 also consist of rare earth-doped optical fibers, thespace required for optical fibers inside the optical amplifier can bekept small.

Next, the action of the optical amplifier with the above configurationwill be explained. Signal light (input light to the optical amplifier)that has been transmitted via the optical transmission path on the inputside passes through the optical isolator 21a and is incident on the rareearth-doped optical fiber 11. If excitation light from the laser lightsource 13 is supplied to the rare earth-doped optical fiber 11, thesignal light is amplified and output to the splitter 15. If excitationlight from the laser light source 13 is not being supplied, the signallight passes through the rare earth-doped optical fiber 11 which is notin the excited state. At this time, the signal light is attenuated bythe rare earth-doped optical fiber 11 instead of being amplified.

Whether or not excitation light is supplied from the laser light source13 to the rare earth-doped optical fiber 11 is determined according tothe input light level as described above. That is to say, if the inputlight level detected by the photodiode 16 is higher than the presetthreshold level, it is judged that the input light contains a signalincluding transmitted information, and the control circuit 19 causes thelaser light source 13 to output excitation light in order to amplifythat input light. On the other hand, if the input light level is lowerthan the threshold level, it is judged that the input light does notcontain a signal to be transmitted. If the input light does not containa signal, it is not necessary to amplify the input light, so the controlcircuit 19 stops driving the laser light source 13. The states of theinput light level and the laser light source 13 are shown in FIG. 3.

As described above, the optical amplifier of this embodiment has therare earth-doped optical fiber 11 in front of the circuit that monitorsthe input light level (the splitter 15 and the photodiode 16); when theinput light contains a signal, the input light level amplified by therare earth-doped optical fiber 11 is monitored.

Next, how the rare earth-doped optical fiber 11 is designed will beexplained. That is to say, the ranges within which the gain whenexcitation light is supplied to the rare earth-doped optical fiber 11,and the loss when excitation light is not supplied, are set, will beexplained.

The gain of the rare earth-doped optical fiber 11 is set to be largerthan the loss that occurs when the input light level is monitored. Theloss that occurs when the input light level is monitored comes aboutbecause the input light is branched (for example, in the ratio 10:1)using the splitter 15. That is to say, the amount of light that isamplified by the rare earth-doped optical fiber 12 is 10/11 of theamount that there would be if the splitter 15 were not used, on accountof the branching of light by the splitter 15, so that a loss of about0.4 dB occurs. Consequently, in this case, the rare earth-doped opticalfiber 11 must have a gain of at least 0.4 dB, so that when the signallight passes through the rare earth-doped optical fiber 11 that lightamount will be amplified by a factor of at least 11/10.

In general, the gain of a rare earth-doped optical fiber depends on itslength, if we assume that the concentration of the erbium that is dopedinto it is fixed. FIG. 4A shows the relation between the length of therare earth-doped optical fiber and its gain. As shown in FIG. 4A, thelonger the rare earth-doped optical fiber, the larger the gain. Inaddition, as shown in FIG. 4B, the gain per unit length of a rareearth-doped optical fiber is almost constant regardless of changes inthe input light level, as long as the input light level does not becometoo high. Consequently, if the wavelength of the input light to beamplified, the wavelength of the excitation light, and the excitationlight power are known, the gain can be determined by adjusting thelength of the rare earth-doped optical fiber.

For example, if branching-off a part of the input light by the splitter15 reduces the light level input to the rare earth-doped optical fiber12 by 0.4 dB, the length of the rare earth-doped optical fiber 11 isadjusted so that the gain of the rare earth-doped optical fiber 11 whenthe excitation light is supplied is larger than 0.4 dB.

Meanwhile, when excitation light is not supplied, the rare earth-dopedoptical fiber 11 does not amplify the input light, but rather attenuatesthe input light. That is to say, a loss occurs. In general, the loss ina rare earth-doped optical fiber to which excitation light is notsupplied is greater than the loss in an ordinary optical fiber, and thelonger the rare earth-doped optical fiber, the greater the loss.Consequently, if the rare earth-doped optical fiber 11 is made too longin order to make the gain greater than a specified value when excitationlight is supplied, then when excitation light is not supplied, the lossin the rare earth-doped optical fiber 11 will become large. That is tosay, if the rare earth-doped optical fiber 11 is made longer thannecessary, then the input light may not be able to pass through whenexcitation light is not supplied.

Here, in order to explain to what extent the loss in the rareearth-doped optical fiber 11 when excitation light is not suppliedshould be suppressed, we assume the configuration of an opticaltransmission system as shown in FIG. 5. In the system shown in FIG. 5,an optical amplifier shown in FIG. 2 is installed at each of the relaynodes; the minimum light level that must be received by each opticalamplifier is predetermined to be, for example, -35 dBm. In other words,in this system, when a signal is being transmitted, if the input lightlevel to each optical amplifier is -35 dBm or greater, it can beguaranteed that the light will be transmitted to the next relay node. Wealso assume that the photodiode 16 can convert the light levelaccurately into an electrical signal if the light level is, for example,-50 dBm or greater.

In the system described above, even if each optical amplifier receiveslight in the order of -35 dBm or greater, it would be possible toaccurately monitor the input light level. Since the splitter 15 branchesoff part of the input light in the ratio 10:1 and the photodiode 16receives 1/11 of the input light, the light level received by thephotodiode 16 is in the order of 10 dB less than the light level outputfrom the rare earth-doped optical fiber 11. Consequently, for example,if we assume that the total loss in the connector 20a and the opticalisolator 21a is 1 dB, and if the design must provide a margin of 3 dB,the loss in the rare earth-doped optical fiber 11 when excitation lightis not supplied must be kept to 1 dB (-35-(-50)-10-1-3=1) or less. Inother words, if the loss in the rare earth-doped optical fiber 11 whenexcitation light is not supplied is defined in this manner, then, evenwhen excitation light is not being supplied, the input light level whena signal is transmitted can be reliably monitored. In general, asdescribed above, the loss in a rare earth-doped optical fiber whenexcitation light is not supplied is proportional to its length.Consequently, in order to set the loss to be at or below a certainlevel, it is sufficient to keep the length of the rare earth-dopedoptical fiber to be at or less than a certain length.

The important points in designing the rare earth-doped optical fiber 11are given below.

(1) The gain of the rare earth-doped optical fiber 11 when excitationlight is being supplied must be greater than the loss that occurs whenthe input light level is monitored.

(2) The loss in the rare earth-doped optical fiber 11 when excitationlight is not being supplied must be smaller than the difference betweenthe minimum light level predetermined in the system in which the opticalamplifier is installed and the smallest light level that can be detectedby the photodiode 16.

If the length of the rare earth-doped optical amplifier 11 is adjustedso that it satisfies the above two conditions, a minimum limit on thelength of the rare earth-doped optical amplifier 11 is given by (1)above and a maximum limit is given by (2) above.

In the above configuration, signal light that arrives after beingtransmitted via an optical transmission path is amplified by the rareearth-doped optical fiber 11, then is guided to a device (the splitter15 and the photodiode 16) that monitors the input light level. At thistime, since the gain of the rare earth-doped optical fiber 11 is greaterthan the loss that occurs in the input light level monitoring device,the S/N (signal-to-noise ratio) of the input signal is not decreased byproviding a device to monitor the input light level. Consequently, thenoise index of the optical amplifier does not become worse.

If the level of the signal light input to the rare earth-doped opticalfiber 11 is limited to the input range shown in FIG. 4B (the unsaturatedregion, or the small signal input region), since the gain does not varywith input light level within that range, the light output level of therare earth-doped optical fiber 11 is proportional to the level of lightinput level to the optical amplifier. In this case, the input lightlevel to the optical amplifier can be computed easily based on theoutput of the rare earth-doped optical fiber 11. In fact, since opticalamplifiers are often installed where they will receive signal light thathas been transmitted over a long transmission path, the input lightlevel to the optical amplifier is normally within the unsaturated regionshown in FIG. 4B.

An optical amplifier with the configuration described above has two rareearth-doped optical fibers, but the excitation light output from asingle light source (the laser light source 13) is supplied to both ofthose rare earth-doped optical fibers, so the power consumption forproviding excitation light does not become very large.

FIG. 6 is a detailed block diagram of the control circuit 19. Asdescribed above, the output light from the rare earth-doped opticalfiber 11 is branched by the splitter 15, and the branched-off light isconverted into an electrical signal by the photodiode 16. Consequently,this electrical signal provides data that indicate the light level ofthe input light to the optical amplifier. The control circuit 19 writesdata indicating the input light level received from the photodiode 16into the buffer 31. The buffer 31 can be, for example, a voltagefollower. The output from the buffer 31 is supplied to the amplifiers 32and 33. The amplifiers 32 and 33 have gain G1 and gain G2, respectively.Here we assume G1<G2. The outputs from the amplifiers 32 and 33 aretransferred to the Vi terminal of the comparator 36 via the switches 34and 35, respectively. The switches 34 and 35 are, for example,constructed using FETs. When the switch control signal that they receiveis "L", they switch ON; when "H" is received they switch OFF. Theseswitch control signals are formed based on the result of a comparison bythe comparator 36.

The reference voltage V1 or V2 is input to the Vc terminal of thecomparator 36, according to the states of the switches 37 and 38. Themanner in which V1 and V2 are determined will be described below. Inaddition, the switches 37 and 38, like the switches 34 and 35, come ONwhen the switch control signal "L", is received, OFF when "H" isreceived.

The comparator 36 compares the voltage applied to the Vi terminal withthe voltage applied to the Vc terminal; if the voltage applied to the Viterminal is higher, "H" is output, while if the voltage applied to theVc terminal is higher, "L" is output. The voltage applied to the Vcterminal of the comparator 36, that is to say the reference voltage V1or V2, is the threshold level to which the input light level iscompared. The comparator 36 outputs "H" if the input light level ishigher than the threshold level, "L" if it is lower. The output of thecomparator 36 is transferred to the switches 35 and 38, and, at the sametime, is inverted by the inverter 39 and then transferred to theswitches 34 and 37. These signals are the switch control signals.

The output light from the rare earth-doped optical fiber 12 is branchedby the splitter 17 and the branched-off light is converted into anelectrical signal by the photodiode 18. This output signal provides dataindicating the light level of the output light from the opticalamplifier. The data indicating this output light level are input to thelaser drive circuit 40. The laser drive circuit 40 contains a powersupply, and determines the output voltage in accordance with the dataindicating the output light level received from the photodiode 18. Inthe present embodiment, the laser drive circuit 40 controls its outputvoltage so that the output from the photodiode 18 will be held constant,so that output light level of the optical amplifier is held constant.When the output from the comparator 36 becomes "L", the action of thelaser drive circuit 40 stops. That is to say, if the input light levelis below the threshold level, the laser drive circuit 40 does notoperate.

The output from the laser drive circuit 40 is fed to the laser lightsource 13 via the switch 41. The switch 41 is controlled by the outputof the comparator 36. When "L" is received it switches ON; when "H" isreceived it switches OFF. When the switch 41 is ON, the laser lightsource 13 outputs excitation light of a power corresponding to theoutput voltage from the laser drive circuit 40. When the switch 41 isOFF, the laser light source 13 is not driven, and excitation light isnot generated. That is to say, if the input light level is higher thanthe threshold level, the laser light source 13 is driven correspondingto the output voltage from the laser drive circuit 40, while if theinput light level is lower than the threshold level, the laser lightsource 13 is not driven.

Now the changes of state of an optical amplifier with the configurationdescribed above, will be explained by referring to FIG. 7 and FIG. 8.The optical amplifier of this embodiment switches between the normalstate and the shutdown state. The normal state is the state in whichexcitation light is input into the rare earth-doped optical fibers 11and 12, and occurs when the input light level is higher than thethreshold level. Meanwhile, the shutdown state is the state in whichexcitation light is not input to the rare earth-doped optical fibers 11and 12, and occurs when the input light level is lower than thethreshold level.

Let us consider the case in which the input light contains a signal,with the optical amplifier in its normal state. The input light level atthis time is, for example, level 1. In FIG. 7, the input light levelappears as the input voltage to the comparator 36. Also, to simplify theexplanation, the gain G1 of the amplifier 32 is assumed to be "1".

Level 1 is larger than the reference voltage V1, and the output of thecomparator 36 becomes "H". For this reason, the output of the amplifier32 is applied to the Vi terminal of the comparator 36, while thereference voltage V1 is applied to the Vc terminal. Also, the laserlight source 13 outputs excitation light.

In the normal state described above, for example, if the input lightceases to contain a signal, the input light level drops to level 2. Atthis time, the output voltage of the photodiode 16 drops. When the inputlight level thus becomes smaller than the reference voltage V1, theoutput of the comparator 36 changes from "H" to "L", the switches 35 and38 switch ON, and, at the same time, the switches 34 and 37 switch OFF.In this state, the output voltage from buffer 31 is applied to the Viterminal of the comparator 36 after being amplified by the amplifier 33(gain G2), and compared to the reference voltage V2. In addition, whenthe output of the comparator 36 becomes "L", the switch 41 switches OFF,as a consequence of which the laser light source 13 no longer outputsexcitation light, and the optical amplifier goes into the shutdown state(the non-excited state).

In the shutdown state, the rare earth-doped optical fibers 11 and 12 arein the non-excited state, so that when input light passes through therare earth-doped optical fiber 11, that input light is not amplified; onthe contrary, it is attenuated. Consequently, the input light level thatis obtained as the output of the photodiode 16 (the output light levelof the rare earth-doped optical fiber 11) is smaller than in the normalstate. In FIG. 7, when input light that does not contain a signal passesthrough the rare earth-doped optical fiber 11 to which excitation lightis not being supplied, the level, assuming that the output light fromthe rare earth-doped optical fiber 11 is amplified at the same gain asin the normal state (gain G1 by the amplifier 32) is expressed as level4. Level 3 is the level obtained assuming that, when input light thatcontains a signal passes through the rare earth-doped optical fiber 11to which excitation light is not being supplied, the output light isamplified at the same gain as in the normal state (gain G1 by theamplifier 32).

In the assumed configuration described above, when the input lightcontains a signal level 3 is detected, while when the input light doesnot contain a signal level 4 is detected. Here, in order to discriminatelevel 3 from level 4 using the comparator 36, it is sufficient to set athreshold level that is intermediate between those two levels, and thenjudge based on whether the measured level is higher or lower than thethreshold level. However, in the configuration assumed above, as shownin FIG. 7, the difference between level 3 and level 4 is small, so thatthere is danger that the above judgment will be erroneous.

In order to solve this problem, in the optical amplifier of thisembodiment, when the optical amplifier switches from the normal state tothe shutdown state, the amplifier that amplifies the output of thephotodiode 16 is switched from amplifier 32 to amplifier 33, so that theoutput of the photodiode 16 is amplified at a gain G2 that is largerthan the gain G1 which is used when the optical amplifier is in thenormal state. As a result, as shown in FIG. 7, level 3 and level 4 areamplified to level 3' and level 4', respectively. If the output of thephotodiode 16 is thus amplified, the difference between level 3' andlevel 4' is large, and it is easy to set the threshold level (thereference voltage V2). As shown in FIG. 7, the reference voltage V2 can,for example, be set to the same order as the reference voltage V1.

Thus, in the optical amplifier of this embodiment, by increasing thegain of the amplifier in the shutdown state, the sensitivity of thephotodiode 16 is essentially increased, so that even if the light levelis in the order of level 3 it can be detected as light in the order oflevel 3'.

If the input light comes to contain a signal while the optical amplifieris in the shutdown state, the input light level increases from level 4'to level 3'. In other words, when the input light level rises above athreshold level, the control circuit 19 judges that the signal lightthat is input includes a signal to be transmitted. In this case, thevoltage applied to the Vi terminal becomes larger than the referencevoltage applied to Vc, and the output of the comparator 36 becomes "H".As a result, the output voltage from the buffer 31 is applied to the Viterminal of the comparator 36 after being amplified by the amplifier 32(gain G1), and that amplified voltage is compared to the referencevoltage V1. In addition, when the output of the comparator 36 becomes"H", the switch 41 switches ON, the laser light source 13 is driven bythe laser drive circuit 40, and the laser light source 13 outputsexcitation light. Then the rare earth-doped optical fibers 11 and 12 gointo the excited state, and the light that passes through them becomesamplified.

FIG. 9 and FIG. 10 are diagrams showing variations of the configurationof the optical amplifier of this embodiment. In FIG. 9 and FIG. 10, thesame numbered symbols have the same functions as described withreference to FIG. 2. In FIG. 9 and FIG. 10, the symbols showing whereoptical fibers are fused together are omitted.

When the optical amplifier shown in FIG. 9 is compared with theconfiguration shown in FIG. 2, the positions of the device that suppliesexcitation light (the laser light source 13 and the optical coupler 14)and the device that monitors the output light (the splitter 17 and thephotodiode 18) have been interchanged.

In the configuration shown in FIG. 10, the device that suppliesexcitation light (the laser light source 13 and the optical coupler 14)is located on the input side of the rare earth-doped optical fiber 11.That is to say, the optical amplifiers shown in FIG. 2 and FIG. 9 areconfigured so that excitation light is supplied from the output sides ofthe rare earth-doped optical fibers 11 and 12 (backward excitation), butthe optical amplifier shown in FIG. 10 is configured so that excitationlight is supplied from the input sides of the rare earth-doped opticalfibers 11 and 12 (forward excitation).

FIG. 11 is a diagram showing another variation of the configuration ofthe optical amplifier of this embodiment. In FIG. 11, the same numberedsymbols have the same functions as described with reference to FIG. 2.In the optical amplifier shown in FIG. 11 the laser light source 52supplies excitation light to the optical fiber 51 via the opticalcoupler 53, and the laser light source 54 supplies excitation light tothe rare earth-doped optical fiber 12 via the optical coupler 55. Theoptical fiber 51 may be the same as the rare earth-doped optical fiber11. The control circuit 56 performs basically the same control as thecontrol circuit 19 shown in FIG. 2. That is to say, the control circuit56 determines the input light level from the electrical signal output bythe photodiode 16, and the output light level from the electrical signaloutput by the photodiode 18. When the input light level becomes lowerthan the threshold level referred to above, the control circuit 56applies control so that light is not output from the laser light sources52 and 54. The control circuit 56 holds the output light level fixed,for example by means of an ALC (Automatic Level Control) function. Theoptical amplifier shown in FIG. 11 has an optical isolator 21c betweenthe optical fiber 51 and the rare earth-doped optical fiber 12.

When the input light does not contain a signal, it is sufficient to stoplight emission from only the laser light source 54 while the laser lightsource 52 continues to emit light. In this kind of configuration, thepower consumption is greater than in a configuration in which both ofthe laser light sources 52 and 54 stop emitting light, but this isadvantageous from the point of view of monitoring sensitivity becauselight that is input into the optical fiber 51 during the shutdown stateis not attenuated.

FIG. 12 is a diagram showing a variation of the configuration of theoptical amplifier shown in FIG. 11. The optical amplifier shown in FIG.12 has a backward excitation configuration in which excitation light issupplied from the output side of the rare earth-doped optical fiber 12.

FIG. 13 is a diagram of still another variation of the configuration ofthe optical amplifier of this embodiment. The optical amplifier shown inFIG. 13 has a dispersion compensation optical fiber 61 between the rareearth-doped optical fiber 11 and the rare earth-doped optical fiber 12shown in FIG. 2 (between the optical fiber 51 and the rare earth-dopedoptical fiber 12 shown in FIG. 11). In FIG. 13, the control circuit isomitted.

The dispersion compensation optical fiber 61 is installed on thetransmission path inside the optical amplifier by means of theconnectors 62a and 62b. In this kind of configuration, a number ofoptical fibers having different dispersion compensation values areprovided, and an appropriate dispersion compensation optical fiber canbe selected and connected according to the transmission path on theinput side of the optical amplifier. The dispersion compensation opticalfiber 61 produces a nonlinear effect depending on the light input level.Consequently, the light input level to the dispersion compensationoptical fiber 61 can be adjusted to have an optimum value, for example,by controlling the light power emitted from the laser light source 52.

FIG. 14 is a diagram of a variation of the configuration of the opticalamplifier shown in FIG. 13. The optical amplifier shown in FIG. 14 has aconfiguration in which, in the optical amplifier shown in FIG. 13,another rare earth-doped optical fiber 63 is installed between the rareearth-doped optical fiber 11 and the dispersion compensation opticalfiber 61. The control circuit 66 drives the laser light source 64 sothat the input level to the dispersion compensation optical fiber 61 isthe optimum level for dispersion compensation, thus applying control sothat the output light level from the rare earth-doped optical fiber 63is held fixed. The output from the laser light source 64 is guided bythe optical coupler 65 and supplied as excitation light to the rareearth-doped optical fibers 63 and 11.

FIGS. 15 to 17 are diagrams of still further variations of theconfiguration of the optical amplifier of this embodiment. These opticalamplifiers are configured so that the optical isolator on the input sideof the rare earth-doped optical fiber 11 (for example, the opticalisolator 21a in FIG. 2) can be eliminated by using connectors having lowreflection as the input side connectors. Note that this optical isolator21a is installed between the rare earth-doped optical fiber 11 and therare earth-doped optical fiber 12.

In FIGS. 15 to 17, the connector 71 is a low-reflection connector thatconnects the input side optical transmission path to the opticalamplifier. The amount of reflection in the connector generally dependson the cross-sectional shapes of the ends of the optical fiberscontained in that connector. It is known that, for example, that aspherical polished shape and a diagonal polished convex spherical shapeare cross-sections which produce low reflection.

Thus, in a configuration in which the optical isolator is removed fromthe input side of the rare earth-doped optical fiber 11, considering thepossibility that the optical isolator itself is a cause of loss, theinput light level to the rare earth-doped optical fiber 11 is increased.For example, if we assume that the loss in the optical isolator is 0.5dB, by removing that optical isolator the input light level to the rareearth-doped optical fiber 11 is increased by 0.5 dB. Consequently, evenif the gain of the rare earth-doped optical fiber 11 is decreased by anamount equal to the loss in the optical isolator, the input light levelcan still be monitored accurately.

The optical band pass filter 72 shown in FIG. 15 and FIG. 16 is a filterthat passes the wavelength components that carry the signal. Forexample, in a light transmission system using signal light in the 1550nm band, the signal is often actually carried on light of wavelength1552 nm or 1557 nm, but the lasers used in 1550 m band lighttransmission systems normally have a peak of light intensity in thevicinity of 1530 nm. In this case, the optical band pass filter 72passes light of the wavelength of, for example, 1545 nm to 1565 nm,while cutting off light of the wavelengths in the vicinity of 1530 nm.In this kind of configuration, a drop in the gain of the rareearth-doped optical fiber caused by emission of light in the 1530 nmband can be prevented.

In FIG. 15 and FIG. 16, the optical band pass filter 72 is placed on theoutput side of the rare earth-doped optical fiber 12, but it can also beplaced between the rare earth-doped optical fiber 11 and the rareearth-doped optical fiber 12. However, if the optical band pass filter72 is placed between the rare earth-doped optical fiber 11 and the rareearth-doped optical fiber 12, it is necessary to use such aconfiguration that the excitation light will not be cut off by theoptical band pass filter 72. An optical notch filter can be used inplace of the optical band pass filter.

The optical amplifier shown in FIG. 15 is of forward-excitationconfiguration, with the excitation light supplied from the input sidesof the rare earth-doped optical fibers 11 and 12.

In the configuration shown in FIG. 16, the splitter 73 branches thelight output from the laser light source 74 in order to supplyexcitation light to the rare earth-doped optical fibers 11 and 12. Theexcitation light branched by the splitter 73 is guided by the respectiveoptical couplers (wavelength division multiplexing couplers) 75 and 76,and supplied to the rare earth-doped optical fibers 11 and 12.Excitation light is supplied from the output side of the rareearth-doped optical fiber 11 and from the input side of the rareearth-doped optical fiber 12.

The optical amplifier shown in FIG. 17 is of such a configuration thatexcitation light is supplied from the output sides of the rareearth-doped optical fibers 11 and 12. Excitation light output from thelaser light source 77 cannot pass through the optical isolator 21a inthe reverse direction, so the configuration is such that the splitter 78and the optical coupler 79 are used to make the excitation light bypassthe optical coupler 21a. The splitter 78 and the optical coupler 79 arerealized by, for example, respective wavelength-division multiplexingcouplers.

In the above embodiments, the configurations were described assumingthat the excitation light stops when the input light level drops below athreshold level, but it is also possible to have configurations in whichthe excitation light power is reduced. In particular, in theconfigurations shown in FIG. 10 and FIG. 15, in a case in which theexcitation light is input to a rare earth-doped optical fiber in onestage and then is incident upon a rare earth-doped optical fiber in asucceeding stage, it is possible that even if the excitation light poweris small, the rare earth-doped optical fiber in the first stage will gointo the excited state. For this reason, if the excitation light poweris reduced when the input light level drops below a threshold level, itis possible to put the rare earth-doped optical fiber in the first stageinto the excited state and obtain gain, while suppressing the powerconsumption. In turn, if gain can be obtained in the rare earth-dopedoptical fiber in the first stage, the transition of the input light fromnot containing a signal to containing a signal can be reliably detected.

In addition, in the above embodiments, configurations in which the inputlight is amplified using rare earth-doped optical fibers have beendescribed, but this invention is not limited to such a configuration; itis also possible to have a configuration in which an auxiliary amplifierhaving gain larger than the loss that occurs in the device that monitorsthe input light level is used in a stage preceding the main amplifierthat amplifies the input light to the desired level.

This invention uses two rare earth-doped optical fibers in successivestages in an optical amplifier having an input light level monitoringfunction. The gain of the rare earth-doped optical fiber in the firststage is made larger than the loss that occurs in the device thatmonitors the input light level, so that the input light level can bemonitored without amplifying the input side loss, contributing toreducing the noise in the optical amplifier and making transmission overlonger distances possible.

In addition, this invention makes it possible to achieve the optimumdispersion compensation to match the transmission path. This alsocontributes to reducing the noise in the optical amplifier and makingtransmission over longer distances possible.

What is claimed is:
 1. An optical amplifier that controls amplificationaction in accordance with an input light level, comprising:an inputmonitor for monitoring the input light level; a first optical fiberamplifier, located on the input side of said input monitor, amplifyingthe input light in accordance with excitation light supplied to thefirst optical fiber amplifier, the first optical fiber amplifieramplifying the input light with a gain that is larger than the loss dueto said input monitor; and a second optical fiber amplifier, located onthe output side of said input monitor, amplifying the light that haspassed through said input monitor, wherein, when excitation light is notbeing supplied to the first optical fiber amplifier, a loss in the firstoptical fiber amplifier is less than the difference between a minimumlight level prescribed in a system in which the optical amplifier isinstalled and a minimum light level that said input monitor can detect.2. The optical amplifier according to claim 1, whereinsaid first andsecond optical fiber amplifiers are both rare earth-doped opticalfibers.
 3. The optical amplifier according to claim 1, furthercomprising:a light source supplying excitation light to said first andsecond optical fiber amplifiers; and light source control means foreither stopping said light source or reducing the output of said lightsource when the input light level detected by said input monitor dropsbelow a preset threshold value.
 4. The optical amplifier according toclaim 3, wherein the sensitivity of said input monitor is increasedwhile said light source control means is stopping said light source orreducing the output of said light source.
 5. The optical amplifieraccording to claim 1 further comprising:output monitoring means formonitoring the output light level of said second optical fiberamplifier; and output control means for controlling the output of saidsecond optical fiber amplifier based on the output light level detectedby said output monitoring means.
 6. The optical amplifier according toclaim 1, wherein at least part of a transmission path between said firstoptical fiber amplifier and said second optical fiber amplifier consistsof a rare earth-doped optical fiber.
 7. The optical amplifier accordingto claim 1, whereinsaid first optical fiber amplifier is in unsaturatedoperation within limits of a specified input dynamic range.
 8. Theoptical amplifier according to claim 1, further comprising:a dispersioncompensation optical fiber which is provided after said first opticalfiber amplifier or before said second optical fiber amplifier.
 9. Amethod of amplifying input light in an optical system,comprising:monitoring the input light level on an input side of a mainoptical amplifier, said monitoring incurring a loss; amplifying theinput light before said monitoring by an optical fiber amplifier inaccordance with excitation light supplied to the optical fiberamplifier, the optical fiber amplifier providing a gain which is largerthan the loss incurred by said monitoring; and halting the amplificationaction of the main amplifier when the input light level is lower than aspecified threshold value, wherein, when excitation light is notsupplied to the optical fiber amplifier, a loss in the optical fiberamplifier is less than the difference between a minimum light levelprescribed in the optical system and a minimum light level that saidmonitoring can detect.
 10. A method of amplifying input light in anoptical system, comprising:amplifying the input light using a firstoptical amplifier in accordance with excitation light supplied to thefirst optical amplifier; monitoring an output light from the firstoptical amplifier with a loss that is smaller than a gain of the firstoptical amplifier; amplifying the output light from the first opticalamplifier using a second optical amplifier; and halting an amplificationaction of at least the second optical amplifier when the monitored lightlevel is lower than a specified threshold value, wherein, whenexcitation light is not supplied to the first optical amplifier, a lossin the first optical amplifier is less than the difference between aminimum light level prescribed in the optical system and a minimum lightlevel that said monitoring can detect.
 11. An apparatus comprising:afirst optical amplifier amplifying an input light in accordance withexcitation light supplied to the first optical amplifier; a monitoringdevice monitoring the amplified input light and thereby causing theamplified input light to incur a loss; and a second optical amplifieramplifying the monitored, amplified input light, wherein the firstoptical amplifier amplifies the input light with a gain that is largerthan the loss caused by the monitoring device, and, when excitationlight is not supplied to the first optical amplifier, a loss in thefirst optical amplifier is less than a difference between a minimumlight level prescribed in a system in which the apparatus is installedand a minimum light level detectable by the monitoring device.
 12. Theapparatus according to claim 11, wherein the first and second opticalamplifiers are rare earth-doped optical fiber amplifiers.
 13. Theapparatus according to claim 12, wherein the monitoring device monitorsa signal level of the input light, the apparatus further comprising:alight source supplying the excitation light to the first opticalamplifier, and also supplying excitation light to the second opticalamplifier; and a controller stopping or reducing the supply ofexcitation light to at least one of the first and second opticalamplifiers when the signal level monitored by the monitoring devicedrops below a threshold value.
 14. The apparatus according to claim 13,wherein the monitoring device has a corresponding sensitivity which isincreased while the controller stops or reduces the supply of excitationlight.
 15. The apparatus according to claim 11 further comprising:outputmonitoring device monitoring the output light level of the secondoptical amplifier; and an output controller controlling the output lightlevel of the second optical amplifier based on the output light leveldetected by the output monitoring device.
 16. The apparatus according toclaim 11, wherein at least part of a light transmission path between thefirst and second optical amplifiers includes a rare earth-doped opticalfiber.
 17. The apparatus according to claim 11, wherein the firstoptical amplifier is an optical fiber amplifier in unsaturated operationwithin limits of a specified input dynamic range.
 18. The apparatusaccording to claim 11, further comprising:a dispersion compensationoptical fiber between the first and second optical amplifiers so thatlight travelling from the first optical amplifier to the second opticalamplifier travels through the dispersion compensation optical fiber. 19.A method for an optical system, comprising:amplifying an input lightwith an optical amplifier in accordance with excitation light suppliedto the optical amplifier; monitoring the amplified input light andthereby causing the amplified input light to incur a loss, saidamplifying the input light amplifying the input light with a gain thatis larger than the loss caused by said monitoring; amplifying themonitored, amplified input light; and halting said amplifying of themonitored, amplified input light when a signal level of the monitoredinput light is lower than a threshold value, wherein, when excitationlight is not being supplied to the optical amplifier, a loss in theoptical amplifier is less than a difference between a minimum lightlevel prescribed in the optical system and a minimum light level thatsaid monitoring can detect.
 20. A method for an optical system,comprising:amplifying an input light with a corresponding gain by anoptical amplifier, in accordance with excitation light supplied to theoptical amplifier; monitoring the amplified input light and therebycausing the amplified input light to incur a loss, the loss beingsmaller than the gain by which the input light is amplified; amplifyingthe monitored, amplified input light; and halting said amplifying of themonitored, amplified input light when a signal level of the monitoredinput light is lower than a threshold value, wherein, when excitationlight is not being provided to the optical amplifier, a loss in theoptical amplifier is less than a difference between a minimum lightlevel prescribed in the optical system and a minimum light level thatsaid monitoring can detect.
 21. A method for an optical system,comprising:a first amplifier amplifying an input light in accordancewith excitation light provided to the first amplifier; means formonitoring the amplified input light and thereby causing the amplifiedinput light to incur a loss, the first amplifier amplifying the inputlight with a gain that is larger than the loss; a second amplifieramplifying the monitored, amplified input light; and means for haltingthe second amplifier from amplifying the monitored, amplified inputlight when a signal level of the monitored input light is lower than athreshold value, wherein, when excitation light is not being provided tothe first amplifier, a loss in the first amplifier is less than adifference between a minimum light level prescribed in the opticalsystem and a minimum light level that said means for monitoring candetect.
 22. An apparatus comprising:first amplifying means foramplifying an input light in accordance with excitation light providedto said first amplifying means; means for monitoring the amplified inputlight and thereby causing the amplified input light to incur a loss; andsecond amplifying means for amplifying the monitored, amplified inputlight, wherein the first amplifying means amplifies the input light witha gain that is larger than the loss caused by the monitoring means,wherein, when excitation light is not being provided to the firstamplifying means, a loss in the first amplifying means is less than adifference between a minimum light level prescribed in a system in whichthe apparatus is installed and a minimum light level that said means formonitoring can detect.
 23. An apparatus comprising:an optical fiberamplifier amplifying an input light in accordance with excitation lightsupplied to the optical fiber amplifier; and a monitoring devicemonitoring the amplified input light and thereby causing the amplifiedinput light to incur a loss, wherein the optical fiber amplifieramplifies the input light with a gain that is larger than the losscaused by the monitoring device, and, when excitation light is notsupplied to the optical fiber amplifier, a loss in the optical fiberamplifier is less than a difference between a minimum light levelprescribed in a system in which the apparatus is installed and a minimumlight level detectable by the monitoring device.
 24. Method formonitoring light in an optical communication system,comprising:amplifying an input light with an optical fiber amplifier inaccordance with excitation light supplied to the optical fiberamplifier; and monitoring the amplified input light and thereby causingthe amplified input light to incur a loss, wherein the optical fiberamplifier amplifies the input light with a gain that is larger than theloss caused by said monitoring, and, when excitation light is notsupplied to the optical fiber amplifier, a loss in the optical fiberamplifier is less than a difference between a minimum light levelprescribed in the optical communication system and a minimum light leveldetectable by said monitoring.